[Shea]

Just a quick question.

Why might one expect CaF2 to be soluble in water ?

[Shea]

It's soluble because it's an ionic salt?

What about ionic salts make them do that?

[Yggdrasil]

Yes, CaF₂ is water soluble because it is an ionic salt.  Ionic salts are generally soluble in water because they consist of positively and negatively charged ions held together by electrostatic attraction.  Water, a polar molecule, has a negatively-charged oxygen which can interact with cations favorably and its positively-charged hydrogens can interact with anions favorably.  Therefore, when placed in water, water can disrupt the electrostatic interactions between cations and anions, replacing these cation-anion interactions with cation-water and anion-water interactions.

[Borek]

Trick is, CaF₂ is very weakly soluble, with pK_so = 10.41. Saturated solution is just about 2.1*10^-4M, while saturated solution of CaCl₂ is over 5M.

[tamim83]

So the moral of the story is, not all ionic solids are soluble in water. You may want to look up the solubility rules and know them pretty darn well too.  These rules do not tell you how soluble though.  So acording to these rules, all halides that do dot contain Silver (I), Hg2(1), or Pb (II) are soluble, so CaF2 would be soluble.  But ase Borek said, it really isn't that soluble. Of course you could always heat up your solution to make it more soluble. 

[Borek]

While we are talking about dissolution it is worth of noting here, that the process of dissolving ions in water, which is usually treated as physical change, is in many ways a chemical one. Most cations are dissolved in form of water complex (hydration being driving source behind decomposition of ionic net in crystals). We are happily talking about complexation of cations with ligands like EDTA, or NH₃, or CN^-, as if cations were freely swimming in the solution, but what is really happening is that these ligands replace water in the complex.